The disclosure relates generally to gas turbines systems, and in particular, to a rich burn combustor for a gas turbine engine.
Hydrocarbon fuel burning processes are widely used in stationary power-generating gas-turbine systems. Combustion by-products which pollute the atmosphere are required to be minimized as part of a growing concern about the quality of the earth's atmosphere. Therefore, combustors for stationary power-generating gas-turbine systems are required to produce low levels of nitric oxides (NO, NO2, N2O, etc., collectively referred to as NOx) and of carbon monoxide (CO). Such emissions may lead to acid rain and other environmental problems. The NOx can result from reactions with atmospheric nitrogen, such reactions being referred to as “thermal” and “prompt” NOx, or with fuel-bound nitrogen (FBN). According to well-supported combustion theory, NOx produced by the “thermal” mechanism is due to atmospheric nitrogen being fixed by the radicals responsible for flame initiation and propagation.
The preponderance of thermal NOx in conventional (fuel and air not premixed) combustors, due to the high temperatures in the turbulent mixing interfaces, has led to water or steam injection for NOx control. In this approach, the injected water or steam absorbs heat, reduces the peak temperatures (to below the NOx forming threshold) and so reduces NOx emission levels. The lower temperatures have the undesirable side effect of quenching CO consumption reactions and so the CO levels increase and combustor life and efficiency are reduced. Thus the water or steam injection technique is not ideal for NOx control.
Powerplant constraints dictate that a stability, turn-down ratio (i.e. power changes corresponding to power demand reductions) and efficiency be similar to those of current equipment. NOx control techniques without water or steam injection are referred to as “dry” combustion. Two dry low-NOx combustion techniques have been suggested: (i) rich-lean staged combustion (originally intended for thermal and FBN NOx control but not successful for the reasons discussed below); and (ii) lean premixed combustion (intended for thermal NOx control).
In rich-lean staged combustion, the combustor is divided into a first zone which is rich (equivalence ratio Φ≅1.3-1.8; note that Φ=1 for stoichiometric conditions, Φ>1 being rich and Φ<1 being lean) and a second zone which is lean. Because of the off-stoichiometric conditions, temperatures in each zone are too low for NOx, (e.g. less than 2780° F.) to form via the “thermal” mechanism.
However in prior art staged systems, the mixing of air with the efflux of the rich zone occurs at finite rates and cannot prevent the formation of hot near-stoichiometric eddies. The attendant high temperatures lead to the copious production of thermal NOx, which is triggered at temperatures above about 2780° F. This has been the experience both in the laboratory and in mainframe (100 MW class) gas-turbine equipment. However, rich combustors are suitable for fuels with significant fuel-bound nitrogen content because the amount of oxygen available to produce FBN NOx is limited.
Lean premixed combustors, which are useful if the fuel does not contain nitrogen, are fueled by a lean (prevaporized, if liquid fuel) premixed fuel-air stream at Φ≅0.7. The ensuing temperatures are uniformly too low (e.g., less than 2780° F.) to activate the thermal NOx mechanism. This forms a lower limit to the minimum NOx obtainable in current hydrocarbon-fueled combustors.
A conventional gas turbine engine includes a compressor for compressing air (sometime referred to as an oxidant as the air has oxidizing potential due to the presence of oxygen), which is mixed with fuel in a combustor and the mixture is combusted to generate a high pressure, high temperature gas stream, referred to as a post combustion gas. The post combustion gas is expanded in a turbine (high pressure turbine), which converts thermal energy from the post combustion gas to mechanical energy that rotates a turbine shaft.
During the process of combustion in a rich combustor, the fuel is consumed in an oxygen deficient environment at relatively low temperature. The high temperature discharge from the combustor may be allowed to expand through a high pressure turbine extracting work from the flow. This work extraction results in a significant cooling of the flow. This fuel rich flow can then be mixed with additional air to consume the unburned fuel in the rich flow stream in a second combustor, and more particularly in a lean combustor. The second burning takes place at a significantly (for thermal NOx formation) lower temperatures. The hot air flow from the second combustor is allowed to expand in downstream turbines extracting additional work. In this type of configuration, the production of NOx is minimized due to the relatively cool temperatures in the rich and lean burning cycles, which temperatures are below the established level for the production of thermal NOx. Prompt NOx is also minimized since CO in the lean cycles tends to be negligible. FBN NOx is minimized because the rich combustor runs with too little oxygen for production of NOx. Additional information regarding this low NOx process of combustion is described in commonly assigned, U.S. Pat. No. RE35,061, issued to Sanjay M. Correa, entitled “Dry Low NOx Hydrocarbon Combustion Apparatus,” which is incorporated by reference herein in its entirety.
By incorporating a secondary lean burn combustor, the post combustion gas is re-combusted after mixing with additional oxygen from the compressor. The re-combusted post combustion gas is expanded in another turbine section (low pressure turbine) to generate additional power. The deployment of the lean combustor and the low pressure turbine therefore utilizes the oxidizing potential of the post combustion gas, thereby increasing the efficiency of the engine.
In an attempt to further increase gas turbine efficiency and specific work, an increase in pressure ratio and firing temperatures may result. This increase in pressure ratio and firing temperatures requires the use of high temperature materials, such as silicon carbide (SiC) in the gas turbine engine. During high temperature operation (in excess of 4000° F.), durability issues exist with regard to chemical attacks of the SiC composite hardware, including but not limited to, combustor liners, domes and turbine blades, by oxygen (O) atoms, hydrogen (H2), hydroxide (OH) radicals and water (H2O) molecules. Environmental barrier coatings may be employed to minimize these chemical attacks, such as water vapor attacks on the SiC components. These coating materials provide satisfactory protection to the SiC components as long as they are not damaged, such as by scratching, or the like, or degraded. Accordingly, during high temperature operation the increase in firing temperature is limited by material capabilities and the NOx emissions.
Accordingly, it is desired to provide for an improved gas turbine engine, capable of operating at increased temperatures.